I mean, I don't find integrating a low degree polynomial over a closed interval to be difficult. If the coefficients are modest, I can do it in my head.
I think any game designer who puts that as a requirement to run a monster in D&D is not a game designer I'd want to use to get content for.
Working out of a I should upcast their 6th level spell to 8th or cast one of their 8th level spells is not something I want to be doing while running an epic fight, no more than I want to be solving integrals. Both are fun to do, but wrong context.
When running that fight, I want stuff to happen, players to be engaged, and my decision:impact ratio to be maximized.